In many technical sectors it is frequently required to seal an opening in a fluid-tight manner, in particular an opening in a fluid-conveying pipe. Generally, a sealing element with dimensions at least equal to those of the opening to be sealed is used and held against the latter. A gasket inserted between the edges of the opening and the sealing element ensures that the required tightness is obtained. Owing to the presence of the gasket, the unevenness in the surface and deformations both of the edge of the opening and of the sealing element can be absorbed to a certain extent. However, it is necessary that these deformations should not exceed certain limits otherwise adequate tightness is not ensured. It is indispensable, therefore, that the sealing element and the edges of the opening to be sealed are machined with a certain degree of precision. The manufacturing costs are therefore relatively high. Moreover, the dimensions of the opening to be sealed must also be relatively precise.